The propagation of errors and faults from a controller to the wider external system that it controls can occur if the primary control system fails. This can result in one or more parts of the external system either generating an illegal response or failing to respond at all. A fault in a primary controller can be caused, typically, by corrupted software, a hardware fault or by operation out of specification. Faults may be propagated externally to the primary controller via those of its output pins which have been configured to an incorrect value. Correct values are required in an appropriate timeframe for the intended operation of the external system. U.S. Pat. No. 7,359,819 discloses a method of testing a driver output by comparing hash values taken from normalised output data with baseline output data and detecting variations between the two. A conventional hash algorithm is used for computing the hash values.